Power plants

ABSTRACT

This invention relates to steam power plants where the generated steam is used to drive a turbine so providing power. In order to reheat the steam at an intermediate point in the turbine, the steam is removed from the turbine, dried if required, reheated in heat exchange with a substantial excess over that which will condense of the live steam, and then returned to the turbine. The live steam is derived from the same source as that used to feed the turbine and, after being used to reheat the steam from the turbine, the excess steam is separated from the condensed water, and both the steam and water are used in the power plant so as not to waste them.

[11] 3,724,212 51 Apr. 3, 1973 [54] POWER PLANTS [75] Inventor: Alan Bell, London, England [73] Assignee: Foster Wheeler John Brown Boilers Limited, London, England [22] Filed: Nov. 26, 1969 [21] Appl. No.: 870,576

OTHER PUBLICATIONS Steam Power Plant Engineering, George Gebhardt; John Wiley & Sons; New York, 1917 (fifth edition). Pgs. 679-684.

Steam Power Plant Auxillaries and Accessories, by Terrell Croft; McGraw-l-lill Book Co. Inc: New York, 1922 (1st edit.) Pgs. 385-389.

Primary Examiner-Martin P. Schwadron Assistant Examiner-Allen M. Ostrager Attorney-Dowell and Dowell [57] ABSTRACT This invention relates to steam power plants where the generated steam is used to drive a turbine so providing power. In order to reheat the steam at an intermediate point in the turbine, the steam is removed from the turbine, dried if required, reheated in heat exchange with a substantial excess over that which will condense of the live steam, and then returned to the turbine. The live steam is derived from the same source as that used to feed the turbine and, after being used to reheat the steam from the turbine, the excess steam is separated from the condensed water, and both the steam and water are used in the power plant so as not to waste them.

9 Claims, 1 Drawing Figure PATENTEDAPR 3 I975 3.724.212

MM g tfzenlor POWER PLANTS BACKGROUND TO THE INVENTION This invention relates to power plants, and in particular steam power plants where steam is used to drive a turbine so providing power.

It is very undesirable, that excessive condensation of the steam should occur in the turbine stages, and particularly the higher pressure stages, because the water droplets can cause severe damage to the turbine blades.

Therefore with conventional fuel fired boilers where the furnace gases have very high temperatures it is the practice to feed superheated steam to the first turbine and to reheat the steam between successive turbine stages.

The problem may not be overcome as easily as this, however, when the steam is provided by a nuclear steam generator since with certain types of reactor, although it can produce very large amounts of hot fluid, this fluid is at a relatively low temperature and it may not be economic or practicable to produce other than saturated steam or steam with a low degree of reheat.

One way of reheating the steam from the first turbine stage is to dry the exit steam from that stage in driers and then to pass that steam in heat exchange with a higher pressure steam, for example, some of the live steam which would otherwise be fed to the turbine inlet. This higher pressure steam condenses and in so doing reheats the exit steam from the turbine. Alternatively the entrained moisture may also be eliminated in the heat exchanger.

When working in this way one has to provide a heat exchanger which is capable of handling the very large volume of steam passing through the turbines, and we have found that a tube and shell heat exchanger in which the live reheating steam is passed through the tubes and the steam to be reheated is passed through the shell across the tubes is suitable. A preferred heat exchanger of this type is described in copending Pat. application Ser. No. 807,732, filed on Mar. 17, 1969.

Because the steam passing through the tubes is condensed, one is faced with another problem. The steam is unavoidably contaminated with traces of so-called non-condensable gases, e.g., oxygen, nitrogen and CO This contamination is particularly evident when the steam comes from a boiling water reactor. Although these non-condensable gases may initially be present as traces, as the steam progressively condenses in the tubes of the heat exchanger, the proportion of the noncondensable gases increases and, once most of the steam has condensed, these non-condensable gases can represent a very substantial proportion of the remaining vapor.

The non-condensable gases naturally accumulate at the outlet ends of the tubes of the heat exchanger where the velocity of the vapor is relatively low, and their accumulation can cause blockage and flow stag? nation with the result that the heat exchanger efficiency is impaired.

One can remove the accumulated gases by periodic venting, but there will still be times between venting when flow stagnation can occur and, what is more important, intermittent venting may be undesirable in that it could adversely affect the stable operation of the turbine.

It is, therefore, an object of the invention to prevent undesirable accumulation of the non-condensable gases within the tubes of the heat exchanger.

According to the invention there is provided a method of operating a power plant in which live steam from a source is fed serially through a number of turbine stages and is reheated between a higher pressure turbine stage and a lower pressure turbine stage by heat exchange with a portion of the live steam from the source, and in which a substantial excess of live steam over that which will condense during the heat exchange is used in the heat exchange step so as to prevent accumulation of any non-condensable gases, and the excess vapor consisting of excess steam together with any noncondensable gases is separated from the condensed water and at least part of the excess separated vapor is fed to the turbine to do useful work therein.

According to another aspect of the invention there is provided a power plant comprising a steam turbine having a number of turbine stages which are serially fed with a supply of steam from a steam source, a tube and shell heat exchanger positioned between higher and lower pressure stages in which the steam passing from the higher to the lower pressure turbine stages is reheated on passing through the shell by heat exchange with a supply of high pressure steam passing through the tubes and derived from the same source as the steam passed through the turbine, a separator for separating condensed water and vapor consisting of excess steam and any non-condensable gases from the tubes of the shell, and means for feeding the separated vapor to the turbine to do useful work therein.

By using the invention, one can ensure that there is always a good flow of reheating steam through the heat exchanger even at the tube outlets and in this way any accumulation of non-condensable gases at regions in the heat exchanger are prevented.

There are a number of uses to which the separated excess vapor can be put. At least some excess vapor is used in the turbine since the loss of the excess live steam might reduce the overall efficiency of the plant.

In one embodiment the excess vapor is fed to the inlet of the high pressure turbine and this is possible if the steam for reheating is withdrawn before the boiler stop valve since the pressure loss between this point and the turbine inlet may be larger than the pressure loss of the steam through the heat exchanger and separator. In another embodiment the excess vapor is introduced into the turbine at an intermediate point. It is also possible to send some of the excess vapor to a high pressure feed heater.

The separated condensed water can be fed to a feed heater so that its heat content is not wasted.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The inventionwill now be described, by way of example, with reference to the accompanying drawing which is a flow diagram of a power plant according to the invention.

The power plant shown in the drawing comprises a steamturbine fed with steam from a steam boiler 10 which can, for example, be a boiling water nuclear reactor. The turbine itself is'in two stages and consists of a high pressure stage 12 and a double low pressure stage 14. Steam from the low pressure stage 14 is condensed in a condenser 16 and returned by a feed pump 18 to the boiler 1(1).

As the steam passes through the stage 12 it loses what, if any, superheat it has and as its pressure drops condensation begins to occur. The design of the whole plant can be, for example, such that at the exit from that stage the steam contains about 12 percent by weight of condensed water droplets. Any higher proportion might lead to considerable damage and erosion of the turbine and so the steam from the stage 12 is first dried by passage through driers 20 and it is then reheated before passing to the low pressure stage 14. As an alternative to the use of the driers 20 one can use an additional heat exchanger.

In the plant shown, the reheating is effected by heat exchange with live steam taken from the boiler 10 before the boiler stop valve 24 in a tube and shell heat exchanger 22, the steam to be reheated passing through the shell and the reheating steam passing through the tubes. A suitable design of heat exchanger 22 is shown in copending Pat. application Ser. No. 807,732, filed on Mar. 17, 1969. In the heat exchanger 22 the live steam is at a higher pressure and temperature than the steam from the stage 12 and so the latter steam is heated while the former condenses.

Since any loss of live steam from the boiler 10 is undesirable because it will reduce the overall efficiency of the plant, one would be inclined simply to feed to the exchanger 22 exactly that amount of steam required for heating and which will condense and no more. This would, however, involve the problem that the traces of non-condensable gases which are inevitably contained in the live steam could accumulate in the tubes of the heat exchanger and result in flow stagnation and blockage, and loss of heat exchange efficiency.

In accordance with the invention this is avoided by feeding an excess of steam to the heat exchanger since this will prevent any accumulation of the non-condensable gases in the tubes of the exchanger. We prefer to pass a substantial excess, such as for example about a 25 percent excess, since this provides a very thorough flushing of the tubes.

The mixture of vapors consisting of excess steam and traces of non-condensable gases and condensed water is passed from the heat exchanger 22 to a separator 26. There the vapors are taken through a line 28 and put to useful work and the water is taken to one of a number of feed heaters 30 where its heat is utilized in heating the feed water.

The vapor in the line 28 can be put to a number of uses. Preferably, however, it is put to work in the turbine. Since it loses very little pressure on passing through the exchanger 22 and separator 26, whereas the live steam passing directly to the turbine stage 12 suffers a somewhat larger pressure drop on passage through the main steam line between boiler and turbine the vapor can be passed directly from the separator 26 to the inlet of the turbine stage 12 along the line A. A1- ternatively it may be desirable to feed the vapor from the separator to an intermediate position in the turbine stage 12 along the line B.

It is not essential, however, for the steam to pass to the turbine, and instead it might be sent to the vapor space of a feed heater such as the high pressure feed 5 above and valves may be provided in the lines A, B and C for this purpose. The particular point in the plant where the vapor is used, however, is chosen so as to give the whole plant the best possible overall efficiency.

By feeding the excess steam to the heat exchanger 22, accumulation of the non-condensable gases in the tubes of the heat exchanger is prevented. The gases are naturally still in the plant but they are now transferred to the shell of the heat exchanger via the turbine and 15 then on to the condenser 16, or to the shell of the feed heater 30 and then on to the condenser where they can be removed conventionally, for example, by taking a small continuous purge, or by means of deaerator. In this way their concentration in the whole plant is kept in equilibrium and at figures which are well within acceptable limits at any particular point.

For the sake of simplicity we have used herein the terms water and steam. These terms are, however, to be construed as meaning any liquid and its vapor unless the context specifically requires otherwise.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

I claim:

1. A method of operating a nuclear power plant comprising the steps:

I. providing live steam from a nuclear reactor source,

II. dividing said live steam into a first part and a second part,

III. feeding said first part to a first turbine having a plurality of stages to provide power output from said first turbine,

IV. exhausting said first part of'said steam from said turbine after said steam has done some useful work,

V. passing said exhaust steam in heat exchange with said second part of said live steam so as to reheat said exhaust steam, the amount of said second part of said live steam being such that there is a substantial excess of vapor consisting of excess steam over that which will condense during said heat exchange together with any non-condensable gases,

VI. thereafter passing said exhaust steam to a second 55 turbine to do further useful work,

VII. separating said excess vapor and condensed water from said second part of said live steam which has been used to reheat said first part, and

VIH. passing at least a part of said separated excess vapor to an intermediate point of the first turbine to do useful work therein.

2. A method according to claim 1 in which at least another part of said separated excess vapor is mixed with said first part of said live steam and fed to said turbine.

3. A method according to claim 1 in which at least another part of said separated excess vapor is added at an intermediate part in said turbine before said steam is removed from said turbine for reheating.

4. A method according to claim 1 in which said separated condensed water is passed in heat exchange with feed water passing to said nuclear source.

5. A method according to claim 1 in which, after separation of said excess vapor, at least another part of said separated excess vapor is passed in heat exchange with feed water passing to said nuclear source.

6. A power plant comprising,

a nuclear reactor means for supplying a source of live steam,

a turbine having a plurality of stages,

a heat exchanger,

means for dividing said live steam from said source and feeding a first part to said turbine and a second part to said heat exchanger,

means for directing the exhaust steam from said turbine to said heat exchanger for reheating the heat exchange with said second part of said live steam and directing said reheated exhaust steam to another turbine for further expansion,

a separator for said second part of said live steam for separating excess vapor consisting of an excess steam together with any non-condensable gases and condensed water from said heat exchanger after said second part has been used for reheating said exhaust steam from said first turbine,

means for passing at least part of said separated excess vapor to an intermediate stage of said first turbine to do useful work therein.

7. A power plant according to claim '6 further comprising a feed water heater to which feed water for said live steam source is fed before passing to said source, and a condensate line leading to said heater from said separator, whereby said feed water is passed in heat exchange with said separated condensed water.

8. A power plant according to claim 6 further comprising a feed water heater to which feed water for said live steam source is fed before passing to said source, and a steam line from said separator to said heater, whereby at least another part of said separated vapor is passed to said feed water heater to heat said feed water.

9. A power plant according to claim 6 in which a steam line is provided from the separator to the steam line leading to said first turbine upstream of said means for dividing the live steam. 

1. A method of operating a nuclear power plant comprising the steps: I. providing live steam from a nuclear reactor source, II. dividing said live steam into a first part and a second part, III. feeding said first part to a first turbine having a plurality of stages to provide power output from said first turbine, IV. exhausting said first part of said steam from said turbine after said steam has done some useful work, V. passing said exhaust steam in heat exchange with said second part of said live steam so as to reheat said exhaust steam, the amount of said second part of said live steam being such that there is a substantial excess of vapor consisting of excess steam over that which will condense during said heat exchange together with any non-condensable gases, VI. thereafter passing said exhaust steam to a second turbine to do further useful work, VII. separating said excess vapor and condensed water from said second part of said live steam which has been used to reheat said first part, and VIII. passing at least a part of said separated excess vapor to an intermediate point of the first turbine to do useful work therein.
 2. A method according to claim 1 in which at least another part of said separated excess vapor is mixed with said first part of said live steam and fed to said turbine.
 3. A method according to claim 1 in which at least another part of said separated excess vapor is added at an intermediate part in said turbine before said steam is removed from said turbine for reheating.
 4. A method according to claim 1 in which said separated condensed water is passed in heat exchange with feed water passing to said nuclear source.
 5. A method according to claim 1 in which, after separation of said excess vapor, at least another part of said separated excess vapor is passed in heat exchange with feed water passing to said nuclear source.
 6. A power plant comprising, a nuclear reactor means for supplying a source of live steam, a turbine having a plurality of stages, a heat exchanger, means for dividing said live steam from said source and feeding a first part to said turbine and a second part to said heat exchanger, means for directing the exhaust steam from said turbine to said heat exchanger for reheating the heat exchange with said second part of said live steam and directing said reheated exhaust steam to another turbine for further expansion, a separator for said second part of said live steam for separating excess vapor consisting of an excess steam together with any non-condensable gases and condensed water from said heat exchanger after said second part has been used for reheating said exhaust steam from said first turbine, means for passing at least part of said separated excess vapor to an intermediate stage of said first turbine to do useful work therein.
 7. A power plant according to claim 6 further comprising a feed water heater to which feed water for said live steam source is fed before passing to said source, and a condensate line leading to said heater from said separator, whereby said feed water is passed in heat exchange with said separated condensed water.
 8. A power plant according to claim 6 further comprising a feed water heater to which feed water for said live steam source is fed before passing to said source, and a steam line from said separator to said heater, whereby at least another part of said separated vapor is passed to said feed water heater to heat said feed water.
 9. A power plant according to claim 6 in which a steam line is provided from the separator to the steam line leading to said first turbine upstream of said means for dividing the live steam. 